(1) Field of the Invention
The present invention generally relates to an apparatus for controlling a heater for heating an oxygen sensor used in an internal combustion engine for measuring an air-fuel ratio in an exhaust gas. More particularly, the present invention is concerned with an apparatus for learning a target heater resistance and controlling the temperature of the oxygen sensor on the basis of the learned target heater resistance.
(2) Description of Related Art
Recently, various control devices have been developed which are directed to improving the output power of an internal combustion engine, reducing fuel consumption or clarifying exhaust gas. Such control devices employ oxygen sensors without exception. As is well known, an oxygen sensor is used for measuring the concentration of an oxygen component contained in the exhaust gas. An oxygen sensor has a sensor element (sense portion) formed of a solid electrolyte or a semiconductor. An output signal of the oxygen sensor depends on the temperature of the sensor element thereof.
It is known that an oxygen sensor having a sensor element made of titania (TiO.sub.2) has an air-fuel ratio (A/F) characteristic as a temperature function of the sensor element as shown in the graph of FIG. 1. The vertical axis of the graph represents the air-fuel ratio, and the horizontal axis thereof represents the temperature of the sensor element. A stoichiometric air-fuel ratio exists between air-fuel ratios a.sub.1 and a.sub.2. When the actual air-fuel ratio is equal to or smaller than the air-fuel ratio a.sub.1 (a rich air-fuel ratio), a large amount of hydro carbon (HC) is contained in the exhaust gas. In contrast, when the actual air-fuel ratio is equal to or larger than the air-fuel ratio a.sub.2 (a lean air-fuel ratio), a large amount of nitric oxide (NO.sub.x) is contained in the exhaust gas. It can be seen from the graph of FIG. 1 that the temperature of the sensor element must be regulated so that it is maintained within the narrow temperature range between T1 and T2 so that the air-fuel ratio of the titania oxygen sensor can be kept within the narrow range between a.sub.1 and a.sub.2 including the stoichiometric air-fuel ratio.
With the above in mind, a conventional oxygen sensor is equipped with a heater, which is subjected to a power supply control so that the value of resistance of the heater becomes equal to a definite resistance value. When the resistance value of the heater is regulated at the definite resistance value, the temperature of the sensor element is also regulated at a constant temperature. Such a power supply control is disclosed in Japanese Laid-Open Patent Application Nos. 57-197459, 60-164241 or 60-202348, for example.
As is well known, the resistance values for different heaters built in oxygen sensors are different from each other. In other words, the resistance values of heaters are within a certain resistance range. This fact causes an error in the temperature control of the oxygen sensor. In order to cancel an error in the temperature control of the oxygen sensor arising from variations in the resistance values of heaters, one may consider that the temperature of the oxygen sensor is controlled by a learning control. During the learning control, the supply of power to the heater is controlled so that the heater resistance value becomes equal to a target resistance value. When an internal combustion engine is in a predetermined stable operating condition, the target resistance value is stored, as a learning value, in a storage unit, such as a battery backup random access memory (hereafter simply referred to as a battery backup RAM). The temperature control is carried out based on the learning value.
By employing the learning control of the heater for the oxygen sensor, it becomes possible to individually determine the target resistance values of the respective heaters on the basis of the respective actual resistance values of the heaters. As a result, it becomes possible to compensate for variations in the heater resistance values.
As has been described above, the target resistance value is stored in the storage unit, such as the battery backup RAM, and used for the future temperature control of the heater. In this case, there is a problem which has to be solved. When the learning value stored in the storage unit is destroyed due to, for example, the battery installed in the vehicle is exchanged for another one. After a new battery is mounted on the vehicle, the target resistance value of the heater is set to an initial resistance value. On the other hand, the temperature characteristic of the oxygen sensor will vary due to variations in the heater resistance value as well as deterioration due to the age of the heater. In this case, the initial resistance value will greatly deviate from an appropriate resistance value based on the present condition of the heater. Due to this fact, it becomes very difficult to precisely detect the oxygen density in the vicinity of the oxygen sensor and precisely set the air-fuel ratio to the ideal air-fuel ratio by an air-fuel ratio feedback control. These problems further cause a catalyzer exhaust stench and deteriorate drivability.